Use Meson summary() function.

Now that we require at least 0.52, we can use Meson's builtin summary printing to display the results of configuration.

Testing Done:
Configured with defaults, and with pixmaps disabled to trigger the warning: https://asciinema.org/a/mV2oxOoVCJNdmrPwgqqUJ3mkU?t=17

Reviewed at https://reviews.imfreedom.org/r/848/